1. Field of the Invention
This invention relates to an apparatus for effectively erasing noise developing in a visible image reproduced from a stimulable phosphor sheet carrying a radiation image stored thereon. This invention particularly relates to an apparatus for effectively erasing noise developing in such a reproduced visible image due to repeated use of the stimulable phosphor sheet in a radiation image information recording and reproducing method wherein the stimulable phosphor sheet is exposed to a radiation to record a radiation image therein and then exposed to a stimulating ray to emit light in the pattern of the stored image, the emitted light is converted to an electric signal, and a visible image corresponding to the radiation image is reproduced by use of the electric signal.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to a stimulating ray such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,315,318 and 4,276,473, U.S. patent application Ser. No. 220,780 and Japanese Unexamined patent publication No. 56(1981)-11395, it has been proposed to use the stimulable phosphor for recording a radiation image of an object for medical diagnosis or the like. Specifically, the stimulable phosphor is first exposed to a radiation to have a radiation image stored therein, and is then scanned with a stimulating ray which causes it to emit light in the pattern of the stored image. The light emitted from the stimulable phosphor upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image of a quality suitable for viewing and diagnostic purposes. In this method, in order to make the wavelength distribution of the stimulating ray different from and far apart from the wavelength distribution of the light emitted from the stimulable phosphor and effectively detect the very weak light emitted therefrom, it is preferred to detect light having a wavelength within the range of 300nm to 500nm emitted from the stimulable phosphor upon stimulation thereof by use of a stimulating ray having a wavelength within the range of 600nm to 700nm. For this purpose, it is desired to employ a stimulable phosphor capable of emitting light having a wavelength within the range of 300nm to 500nm upon stimulation thereof by a stimulating ray having a wavelength within the range of 600nm to 700nm, as described in U.S. Pat. No. 4,258,264.
In the radiation image recording and reproducing method described above, the final visible image may be reproduced in the form of a hard copy or may be displayed on a cathode ray tube. The stimulable phosphor sheet used in this method may be in any of various forms such as a panel, drum or the like, which are herein generally referred to as sheets. For economical reasons, it is desirable that the stimulable phosphor sheet be used repeatedly in many separate radiographic operations.
In order to reuse the stimulable phosphor sheet, it is necessary that the stimulable phosphor sheet to be reused be made completely free from the previously stored radiation image. Theoretically, the radiation energy of the radiation image stored in the stimulable phosphor sheet should disappear when the sheet is scanned with a stimulating ray of a sufficient intensity to cause light to emit therefrom in the pattern of the stored radiation image in the course of the radiation image recording and reproducing process as described above. Actually, however, the stored radiation energy cannot be completely eliminated only with the stimulating ray used to scan the stimulable phosphor sheet during the aforesaid process. Thus a part of the previously stored radiation image remains in the reused stimulable phosphor sheet and inconveniently causes noise to occur in the visible image reproduced from the reused stimulable phosphor sheet. In order to successfully reuse the stimulable phosphor sheet, any such residual radiation image thereon must be erased completely before reuse.
Further, a stimulable phosphor contains a trace of radioactive isotopes such as .sup.226 Ra and .sup.40 K, which emit radiations and cause the stimulable phosphor sheet to store the emitted radiation energy even when the sheet is not being used in radiography. These traces of radioactive isotopes also constitute a cause of the noise developing in the reproduced visible radiation image. Furthermore, a stimulable phosphor sheet is also affected by environmental radiations such as cosmic rays and X-rays emitted from other X-ray sources and stores the energy thereof. These types of radiation energy (hereinafter referred to as fog) undesirably stored in the stimulable phosphor sheet also cause noise to appear in the visible radiation image reproduced from a reused stimulable phosphor sheet and, therefore, must be erased before reusing the stimulable phosphor sheet.
In order to avoid noise occurring in the reproduced visible image due to the noise originating from the radiation image previously stored in a stimulable phosphor sheet and due to the fog developing during the storage of the sheet, the Applicant has proposed in his Japanese Unexamined patent publication No. 56(1981)-11392 to stimulate the stimulable phosphor sheet by use of light of wavelengths including the wavelength range of the stimulating ray for the phosphor before storing a radiation image in the stimulable phosphor sheet, thereby to discharge the detrimental radiation energy therefrom to an acceptable extent.
In the above-mentioned noise erasing method, a stimulating ray having a wavelength within the range between visible light and infrared regions is employed, and the light source used for noise erasing is a tungsten-filament, halogen, or infrared lamp emitting light within the range between visible light and infrared rays.
However, it was found that, when the light source described above is used to erase noise on the stimulable phosphor sheet, heat waves such as infrared rays are emitted from the erasing light source to the stimulable phosphor sheet, and consequently, the temperature of the stimulable phosphor sheet rises during noise erasing, resulting in deterioration of the binder contained in the stimulable phosphor sheet for binding the stimulable phosphor. Thus, as the stimulable phosphor sheet is used repeatedly for recording radiation images thereon and erasing is conducted repeatedly thereon, a part of the stimulable phosphor peels from the sheet. This problem is aggravated particularly when the temperature of the stimulable phosphor sheet rises to 60.degree. C. or more.
One solution to the above-mentioned problem is to position the erasing light source far from the stimulable phosphor sheet in order to reduce heat applied from the erasing light source to the stimulable phosphor sheet, and use a large-scale cooling apparatus. In this case, however, the size of the erasing apparatus becomes large, necessitating a large space for installing it. Further, since the erasing light source is spaced far away from the stimulable phosphor sheet, loss of the illuminance applied to the phosphor sheet increases, and it becomes necessary to use an erasing light source having higher power. Consequently, power consumption of the erasing apparatus increases greatly. Furthermore, this method is uneconomical since the power is wasted for the purposes other than the emission of the stimulating ray effective for erasing.